Abstract

Objectives This study aimed to determine thermoregulatory and cardiovascular responses as well as the occurrence of heat illness in children exercising outdoors in physical education class under hot and humid climate. Little information regarding this issue under real-life situation is available, especially in the Southeast Asia.

Design Analytical, prospective descriptive study.

Setting A primary school in Bangkok, Thailand.

Participants A total of 457 schoolboys (aged 5.5–12 years) were observed while exercising outdoors during their physical education classes throughout the academic year of 2009, including semester 1 (between July and September 2009) and semester 2 (between November 2009 and February 2010).

Results Outdoor physical activity consisted of skill practice (duration 24.11±11.04 min, intensity <3 metabolic equivalent of tasks) and playing sports (duration 11.48±5.53 min, intensity 2.6–8.8 metabolic equivalent of tasks). After exercise, tympanic temperature increased by 0.66±0.41°C. There were 20 (4.4%) students whose ear temperature exceeded 38°C, 18 of whom did not consume water. The RR of increasing body temperature up to 38°C in overweight students was 2.1-fold higher than normal-weight students. The per cent change in mean arterial pressure and heart rate increased by 20.16±15.34% and 23.94±19.78%, respectively. Sweat and dehydration rates were 391.16±186.75 ml/h and 0.63±0.26%, respectively. No evidence of heat illness was found. Wet bulb globe temperatures of semesters 1 and 2 were 29.95±1.87°C and 28.32±2.39°C, respectively.

Conclusions There is an increased risk for heat illness during outdoor activities in physical education class in primary school children, especially those who are overweight and have poor hydration status.

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Article summary

Article focus

While children are exercising outdoors in their physical education classes under tropical climate, their thermoregulation and cardiovascular functions may be affected, leading to development of heat-related illness.

Key messages

This study reports the physiological responses and the occurrence of heat illness in primary school children during outdoor activities in physical education class in a tropical climate.

Such information could have important implications for design of an optimal programme as well as prevention of heat illness in physical education classes under hot and humid environment.

Strengths and limitations of this study

To our knowledge, this is the first published report from Southeast Asia to provide descriptive information obtained from a large study population on heat illness surveillance and the physiological responses of primary school children under real-life situation while exercising outdoors during their physical education class.

The limitation of this study includes the measurement of core temperature changes and timing of the heart rate assessment. Ear temperature measurement was taken in this study because it is easy to obtain the children's cooperation. Hence, ear temperature is more practical than rectal temperature measurement for such a field-based study. Another limitation is the time taken for heart rate samples. The results of this study may not speak for the entire population because convenience sampling was used.

Introduction

When exercising in a hot environment, thermal stress imposes physiological strains, including thermoregulation, cardiovascular functions and subjective responses on the body to a greater extent than during regular exercise.1–4 The body has to provide blood flow for increasing metabolism of muscles while increasing skin blood flow and providing fluids for sweating.1 When these simultaneous demands cannot be met, a cascade of events occur, ultimately resulting in circulatory failure and a rise in core temperature to lethal levels.1 This leads to heat illnesses, varying from heat oedema, heat rash, heat syncope, heat cramps, heat exhaustion and heat stroke.1,5–11 Heat exhaustion is a condition in which core temperature rises to 38°C–40°C. Heat stroke is the most severe form with core temperatures of >40°C.9

A retrospective study in the US emergency departments during the period 1997–2006 found that patients aged 19 years or younger accounted for the largest proportion of exertional heat-related illness, the majority of which were associated with performing a sport or exercising.12 In Southeast Asia, there were 27 adult patients admitted for exertional heat stroke to the Medical Unit, Toa Payoh Hospital, Singapore from January 1984 to January 1987.13 Also, eight cases were seen during the hot summer of 1987 at Pramongkutklao Hospital, Bangkok, which represented the first report of this syndrome in Thailand.14 However, reports in children have not been published from Southeast Asia. It is believed that children are at a higher risk for heat-related problems than adults during exercise due to a number of physiological differences. Children have a greater surface area-to-body mass ratio for heat absorption, a greater metabolic heat production per mass unit and a lower sweating capacity.9,15 However, these concepts have recently been contradicted by the findings that no thermoregulatory differences exist between children and adults.16–19 The risk of heat illness may be explained by exposure and social/behavioural differences.20 Children frequently do not drink enough to replace fluid loss during exercise.15,21,22 A study in 10–12-year-old boys revealed that children developed dehydration due to insufficient drinking while exercising in hot climate.21 Another study at a summer sports camp found that children experienced minimal to significant dehydration. Yet the level of hydration knowledge did not correlate with hydration status.22 Additionally, they are more likely to be exposed to outdoor physical activities. Children tend to attend outdoor summer sports camps and spend more time outside.20 For example, in physical education class, participation is mandatory and activities are often conducted in the late morning and afternoon, the hottest period of the day. Thus, the susceptibility to heat illness may be raised if the class takes place outdoors. With the influence of global warming, it is predicted that the incidence of heat illness will also become more prevalent.23,24

Most of the previous investigations on prepubertal children exercising in the heat have been conducted indoors under controlled climatic conditions to determine the effects of maturational and gender differences.21,25–30 Little information on those exercising in field-based environment is available, especially in a tropical region. Consequently, this study aimed to determine cardiovascular and thermoregulatory responses as well as the occurrence of heat-related illness in primary schoolboys during outdoor exercise in their physical education classes under tropical climate. This information is required as the basis for preparing an optimal programme and providing particular care and prevention of heat illness in physical education class.

Methods

Sample

An analytical prospective descriptive study was conducted using convenience samples. Participants were schoolboys from grades 1–6 of the Chulalongkorn University Demonstration Elementary School, located in Bangkok. The study was conducted during the academic year of 2009, which included the first (between July 2009 and September 2009) and second semesters (between November 2009 and February 2010). The survey was conducted by distributing questionnaire, consent form, assent form and patient information sheets to all 778 schoolboys in the school. Four hundred and ninety-six of 778 students were allowed to participate in the survey by their parents. As 39 students did not participate in the outdoor exercise, they were excluded from the study. Therefore, only 457 surveys were collected in this study (n=108, 89, 88, 73, 68 and 31 for grades 1–6, respectively). All students were healthy on the day of the survey, had no history of heat-related illness and were able to understand the questionnaire. Informed written consent and assent were obtained from the students and their parents. The protocol was approved by Institutional Review Board of the Faculty of Medicine, Chulalongkorn University.

Measurement

The students were observed while performing activities outdoors in their physical education classes conducted in the late morning (09:40–11:40 h) or in the afternoon (12:40–15:40 h). Data before and after outdoor exercise were recorded only once for each student. The students were allowed to drink water ad libitum without encouragement and urinate during their physical education classes.

Exercise duration and intensity

Types and duration of outdoor activities performed during physical education class were recorded by using video camera (Sony Handycam DCR-HC46; Sony Corporation, Tokyo, Japan). The intensity of each physical activity, which started with skill practice, followed by playing sports, was determined as metabolic equivalent of task (MET) value using the compendium of energy expenditures for youth given by Ridley et al (2008).31

Thermoregulatory and cardiovascular responses

Ear temperature was measured at the beginning and within 5 min after the completion of exercise5 using infrared temperature sensor for ear temperature (Terumo ear thermometer EM-30CH; Terumo Corporation, Tokyo, Japan). Calibration of the instrument was done before its use by comparing with rectal core temperature measurements in five male volunteers, aged 16–17 years, by using a Biopac MP100 system with SKT100C transducer module and a thermistor probe (TSD102A; Biopac Systems Inc, Santa Barbara, CA, USA) with 1.7 mm of diameter. The thermistor probe was wrapped by plastic film and lubricant gel was applied before inserting to the rectum at a depth around 10 cm. The software for calibrating and collecting data was AcqKnowLedge V.3.8. The tympanic measurements showed a range of 0.1°C–0.2°C lower than the rectal temperature.

Cardiovascular parameters including blood pressure and heart rate were measured at the beginning and immediately upon completion of the respective activities by using digital sphygmomanometer (ES-H55; Terumo Corporation).1 In each exercise period, three to six students were assessed for these parameters by three examiners; three students immediately post-exercise and the rest one after the other. The time taken for heart rate samples to be collected was not more than 2 min.

Hydration status

The students emptied their bladders before the beginning of exercise. Then, they were weighed wearing only their underwear by using an electronic scale accurate to 20 g (Yamato DP-6100GP; Yamato scale Co., Ltd, Akashi, Japan). A batch of 600-ml bottles of water with the student's name was provided for each student to be used without encouragement during exercise. At the end of the exercise, the total fluid intake was measured and urine was collected. Finally, each student was weighed again wearing only his underwear after sweat was wiped from his body. Sweat loss was calculated from the change in body weight plus total fluid intake minus urine output. Sweat rates were then determined as sweat loss per unit of time (in millilitres per hour). This calculation does not take into account the weight loss due to irreversible fuel oxidation and respiratory fluid loss since it was assumed that these would not differ between trials.

Occurrence of heat-related illness

During the exercise period, students were observed for symptoms and signs that may relate to heat illness. These included (1) heat oedema: mild oedema of hands, feet and ankles; (2) heat rash: pruritic rash and papulovesicular skin eruption over the clothed area; (3) heat cramps: painful muscle contractions; (4) heat syncope: loss of postural control, dizziness and generalised weakness; (5) heat exhaustion: profuse sweating, weakness, clammy skin, dizziness, malaise, fatigue, nausea, vomiting and headache and (6) heat stroke: hot skin with or without sweating, confusion, ataxia, irritability and coma.9,10 Data of the students who suffered from heat-related illness would be recorded in the incident report form. The image of the event was recorded by using video camera so the physician can verify the diagnosis.

Analysis

Quantitative data were expressed as means ± SD. Cardiovascular responses and change in body weight were presented as per cent change. Data analysis before and after the exercise period for body temperature, heart rate, blood pressure and body weight was done by using the paired t test. As semesters 1 and 2 were during rainy and winter seasons, respectively, the differences between the two semesters were determined for ear temperature, cardiovascular response, hydration status and climatic conditions by using the independent samples t test. An α level of 0.05 was used to determine statistical significance. All statistical analyses were performed by using Statistic Package for the Social Sciences (SPSS for Windows V.17.0).

Results

A total of 457 students participated in this study without any missing data for each variable of interest. Data collection was done in 175 students from 37 period classes during the first semester and 282 students from 72 period classes during the second semester.

Baseline characteristics of the students are shown in table 1. Assessment of body weight status was determined by using Thai national growth chart. As body mass index references for Thai children are not available, weight for height reference has been used. Overweight is defined as a weight for height of +1.5 SD to +2 SD, corresponding to body mass index of 17.99–22.42 for the height ranging from 107 to 160 cm.32 It was revealed that 128 of 457 (28.0%) students were overweight.

At the high exercise intensity of 8.8 MET, duration of playing sports was 11.43±5.23 min and WBGT was 28.35±2.01°C. Sports with such high intensity included chair-ball (n=45; first graders=24, second graders=21), soccer (n=110; first graders=30, second graders=41, third graders=22, fourth graders=9 and sixth graders=8) and mini-rugby football (n=1, third grader).

Table 2 presents the exercise intensity, ear temperature change, cardiovascular responses, hydration status and climatic conditions during outdoor exercise in the physical education class of the primary schoolboys during the first and second semesters.

Exercise intensity, ear temperature change, cardiovascular responses, hydration status and climatic conditions during outdoor exercise in physical education class of the primary schoolboys during the first and second semesters

The maximal heat stress level was found during an afternoon class in the first semester (WBGT of 33.85 and relative humidity of 48.50%), where five students (fifth graders) were observed while playing handball. The peak responses post-exercise during this class included ear temperature of 37.1°C, and per cent changes in systolic and diastolic blood pressure and heart rate of 56.98%, 84.75% and 13.89%, respectively. The sweat rate was 642.37±193.33 ml/h and fluid intake was 198.20±7.68 ml

Significant difference (p<0.05) in sweat rate was found between first semester morning (370.07±182.15 ml/h, n=49) and afternoon sessions (483.06±232.33 ml/h, n=126). The sweat rate of overweight students throughout the year was 437.97±179.27 ml/h. It was significantly (p<0.05) higher in semester 1 (494.05±208.50 ml/h, n=60) than that in semester 2 (388.49±131.66 ml/h, n=68).

Only 18 students consumed water, an average of 212.89±89.65 ml during the classes, two of whom had a rise in ear temperature up to 38°C. There were two students (first and fourth graders) whose body weight decreased by 2.10%.

There were 20 (4.38%) students whose ear temperature rose to a value exceeding 38°C, with a maximum of 38.7°C. The body temperature of these boys increased by 1.37±0.45°C from 36.78±0.37°C to 38.15±0.20°C. Table 3 presents conditions found in these students. These include WBGT, body weight, water consumption, exercise intensity and exercise period. The incidence of increasing body temperature up to this level in overweight and normal-weight students was 7.0% (nine of 128 students) and 3.3% (11 of 329 students), respectively. Thus, the RR of an increase in body temperature up to 38°C in overweight students was 2.1-fold higher than normal-weight students. However, no students experienced heat-related illness during the observation.

Conditions found in 20 students whose body temperature exceeded 38°C or above during outdoor exercise in physical education class

Discussion

Physical education is important for children because engaging in physical activity will provide them with life-long better health.33 There is a recommendation that children in primary school should spend 150 min on physical education each week, ranging from moderate to vigorous for at least 50% of the physical education time.34 Our main concern is that if the class is held outdoors in intense heat, the children may become vulnerable to heat injury. Therefore, in order to prepare an optimal physical education programme, particular care to quantity and intensity of the exercise should be taken to prevent heat illness.

This is the first published report from Southeast Asia to provide descriptive information on the physiological responses of primary school children under real-life situation while exercising outdoors during their physical education class in hot and humid environment.

The climatic conditions in Bangkok are moderately hot and humid. The students of this study were exposed to high levels of heat stress throughout the academic year, given that the WBGT level was above 28°C, which the American College of Sports Medicine has defined as a ‘very high risk for heat exhaustion and heat stroke’.35 Even more disturbing, the WBGT level of the first semester during July to September was above 29°C, which is the critical level defined as ‘cancel all athletic activities’ by the American Academy of Pediatrics.15 Moreover, WBGT rates in the afternoon were higher than those in the late morning. Therefore, the high values of heat stress index, especially in the afternoon of the first semester, seem to pose a risk in having heat-related illnesses to the students when exercising outdoors. However, this study demonstrated no significant differences in any variables post-exercise except for sweat rate, which was higher in the afternoon than in the late morning during the first semester. Interestingly, under the maximal WBGT of 33.8°C, the students were able to maintain effective thermoregulation by heat loss through sweating, thus, when coupled with fluid consumption, making them withstand such the heat stress with exercise.

In this study, the students were able to maintain adequate cardiovascular function while performing physical activities outdoors. The present study demonstrated that the children had considerably lower heart rate responses compared with previous reports. In the study of Inbar et al (2004),36 prepubertal boys exhibited the per cent change in heart rate of 137.66% when performing a cycling session at the intensity level of 50% VO2max for 85 min in 41°C environment and relative humidity at 21%. Similarly, Rivera-Brown et al (2006)17 found that the per cent change of heart rate was 87.5% in prepubertal girls cycling for 60 min at 60% VO2max in a hot and humid outdoor environment (ambient temperature of 33.7±0.4°C, WBGT 30.0±0.3°C and relative humidity of 53.9±2.4%) with energy drinks to prevent dehydration. The large changes in heart rate obtained from these studies may be due to longer duration and higher intensity of exercises. On the other hand, in our study, the intensity of playing sports was moderate to vigorous, but most of the time, during the physical education class, skill practice was implemented, which was of mild intensity. The lower heart rates could also be the result of the timing of the assessment post-exercise, a significant limitation of the study. Of 457 students, 306 students were assessed immediately and 151 were done within 2 min.

One of the limitations of the study is that we did not take rectal temperature as a measure for core temperature. However, ear temperature measurements can also predict trends in core temperature.37,38 Comparative studies in children indicate that ear and rectal temperature have strong correlation39–42 with the pooled mean difference of 0.29°C.41 In our study, ear temperature measurement was taken because it is more convenient, less time-consuming and, most importantly, easier to obtain the children's cooperation. Hence, ear temperature is more practical than rectal temperature measurement for such a field-based study.

In the present study, there were 95.6% of the students (437 of 457 students) whose ear temperatures were below 38°C. However, there was a risk of heat illness in the children because approximately 4% of the students (20 of 457 students) had a rise in body temperatures of up to 38°C. These students, if exercising for a longer period of time, may have a tendency to develop heat exhaustion, which can lead to heat stroke. However, the students in our study did not exhibit symptoms of heat illness. As shown in table 3, it was found that a larger proportion of these 20 students exercised at WBGT ≥29°C, did not consume water, played soccer/chair-ball/athletics that were of moderate-to-vigorous intensity or exercised in the afternoon. Moreover, the risk of increasing body temperature up to 38°C in overweight students appeared to be approximately twofold higher than normal-weight students. These data support the findings of earlier investigations on the factors affecting heat tolerance capabilities.1,43–45 Heavier children exhibited higher core temperatures and had a lower environmental limits compared with normal-weight children when exercising in the heat.43–45 Core temperature increases in proportion to exercise intensity.1 Results of this study are consistent with previous reports that the worst time of the day to exercise is between 10:00 and 18:00, the hottest period.46,47

Hydration status is one of the major factors in improving performance and limiting heat illness in a hot environment when exercising.1 Dehydration rate due to fluid loss up to 2% of bodyweight can result in performance decrements during exercise.48 Furthermore, dehydration because of fluid loss can occasionally be as high as 6%–10% of the person's bodyweight and appears to be one of the most common risk factors for heat illness in patients.48 Core body temperature has been shown to increase by an additional 0.15°C–0.2°C for every 1% of bodyweight lost to dehydration during exercise.46

The present data showed a higher sweat rate in semester 1 than in semester 2. This can be explained by a higher WBGT and relative humidity in semester 1. As a result, the capability of heat loss by radiation, convection and evaporation is reduced to a greater extent, causing higher sweat rate. Noticeably, most of the students (96.1%) in this study did not consume water. This finding demonstrates that children did not recognise the need to replenish fluid loss during exercise. However, degree of dehydration in our students was mild, suggesting that their sweat rate was not excessive (391.16±186.75 ml/h). In the study of Inbar et al (2004),36 the sweat rate of prepubertal boys was 342±14 ml/h, which was lower than in the young adults and older men. These findings are in line with previous studies reporting relatively low sweat rate in children.49,50

Although the results of this study cannot speak for the entire population, it can be concluded that primary schoolboys may exhibit adequate cardiovascular and thermoregulatory responses during outdoor physical activity, with an intensity of 2.5–9 MET and exercise duration of about 30 min, in physical education class under hot and humid conditions of WBGT 28°C–30°C. However, there is an increased risk for heat illness, especially in overweight children and those with poor hydration status. It is recommended that climatic conditions be measured before starting physical education class. Children should be informed of the importance of drinking and encouraged to drink enough water before and during exercise. The amount of water consumed can be determined in relation to sweat rate, which, based on the results from this study, is approximately 400 ml during an hour of exercise under WBGT ranging of 28°C–30°C. Teachers should allow students to have frequent drinking breaks and prepare modified play-to-rest ratios as well as inclusion of more shaded or ventilated areas. Immediate medical attention should be provided for student's health and safety. Even though this research did not demonstrate any incidence of heat illness in school children participating in physical education class but results were implied only in a city, which may obscure such findings. Further investigation in suburban and rural areas is still needed.

Acknowledgments

The authors thank the schoolboys of Chulalongkorn University Demonstration Elementary School and their teachers and parents for their support.

Contributors JS contributed to conception and design, field study measurements, data analysis, drafting the article and final approval of the version to be published. SS and CP carried out the field study measurements and were involved in acquisition of data.

Funding This study was supported by the Ratchadapiseksompotch Fund, Faculty of Medicine, Chulalongkorn University (Grant No. RA 35/52).

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